A laser diode driver circuit is implemented from two current drivers, a switching circuit, a mirror circuit and one transistor which adjustably sets the ON and OFF bias currents for the laser diode.
A laser diode driver must be adjustable to drive a diode with a low current bias level when the diode is OFF, and a high current level when it is ON. The circuit must be able to handle enough current when driven by a low voltage power supply, and have few components. Operational amplifiers with surrounding circuits are sometimes used but it is difficult to get a single amplifier to output two adjustable current levels. An improved circuit is needed.
The circuit contains two current drivers which generate equal currents which are adjustable to the ON current of the diode. In the ON mode, all of the current from one of the drivers is forced by a control transistor to go through the diode. In the bias (OFF) mode a slight amount of adjustable current from the same control transistor is used determine how much current goes to the diode. Thus, a single control transistor can be used to control the two current levels, by turning the control transistor fully OFF in one case and making the transistor adjustable to the bias current in the other.